The present invention relates to nontoxic gas generating compositions which upon combustion, rapidly generate gases that are useful for inflating occupant safety restraints in motor vehicles and specifically, the invention relates to nonazide gas generants that produce combustion products having not only acceptable toxicity levels, but that also exhibit a relatively high gas volume to solid particulate ratio at acceptable flame temperatures. Additionally, the compositions of the present invention readily ignite and sustain combustion at burn rates heretofore thought to be too low for automotive airbag applications.
The evolution from azide-based gas generants to nonazide gas generants is well documented in the prior art. The advantages of nonazide gas generant compositions in comparison with azide gas generants have been extensively described in the patent literature, for example, U.S. Pat. Nos. 4,370,181; 4,909,549; 4,948,439; 5,084,118; 5,139,588 and 5,035,757, the discussions of which are hereby incorporated by reference.
In addition to a fuel constituent, pyrotechnic nonazide gas generants contain ingredients such as oxidizers to provide the required oxygen for rapid combustion and reduce the quantity of toxic gases generated, a catalyst to promote the conversion of toxic oxides of carbon and nitrogen to innocuous gases, and a slag forming constituent to cause the solid and liquid products formed during and immediately after combustion to agglomerate into filterable clinker-like particulates. Other optional additives, such as burning rate enhancers or ballistic modifiers and ignition aids, are used to control the ignitability and combustion properties of the gas generant.
One of the disadvantages of known nonazide gas generant compositions is the amount and physical nature of the solid residues formed during combustion. The solids produced as a result of combustion must be filtered and otherwise kept away from contact with the occupants of the vehicle. It is therefore highly desirable to develop compositions that produce a minimum of solid particulates while still providing adequate quantities of a nontoxic gas to inflate the safety device at a high rate.
It is known that the use of ammonium nitrate as an oxidizer contributes to the gas production with a minimum of solids. To be useful, however, gas generants for automotive applications must be thermally stable when aged for 400 hours or more at 107° C. The compositions must also retain structural integrity when cycled between −40° C. and 107° C.
Generally, gas generant compositions using ammonium nitrate are thermally unstable propellants that produce unacceptably high levels of toxic gases, CO and NOx for example, depending on the composition of the associated additives such as plasticizers and binders. Known ammonium nitrate compositions are also hampered by poor ignitability, delayed burn rates, and significant performance variability. Several prior art compositions incorporating ammonium nitrate utilize well known ignition aids such as BKNO3 to solve this problem. However, the addition of an ignition aid such as BKNO3 is undesirable because it is a highly sensitive and energetic compound.
Yet another concern is the pressure requirements for complete combustion of various nonazide compositions containing phase stabilized ammonium nitrate. For certain compositions containing phase stabilized ammonium nitrate, the inflator must be manufactured with a more robust design, such as heavier and thicker walls, to accommodate the relatively greater pressure needed to sustain combustion and minimize the potential for performance variation. This adds to the raw material requirements and to the manufacturing complexity. A reduction in the pressure requirements would therefore constitute a substantial improvement in the art.